229 research outputs found
The Australia Telescope campaign to study southern class I methanol masers
The Australia Telescope Compact Array (ATCA) and the Mopra facility have been
used to search for new southern class I methanol masers at 9.9, 25 (J=5) and
104 GHz, which are thought to trace more energetic conditions in the interface
regions of molecular outflows, than the widespread class I masers at 44 and 95
GHz. One source shows a clear outflow association.Comment: 2 pages, 1 figure (composed from 3 files), to appear in proceedings
of IAU Symposium 242 "Astrophysical masers and their environment" (eds. J.
Chapman and W. Baan
Regular spatial structures in arrays of Bose-Einstein condensates induced by modulational instability
We show that the phenomenon of modulational instability in arrays of
Bose-Einstein condensates confined to optical lattices gives rise to coherent
spatial structures of localized excitations. These excitations represent thin
disks in 1D, narrow tubes in 2D, and small hollows in 3D arrays, filled in with
condensed atoms of much greater density compared to surrounding array sites.
Aspects of the developed pattern depend on the initial distribution function of
the condensate over the optical lattice, corresponding to particular points of
the Brillouin zone. The long-time behavior of the spatial structures emerging
due to modulational instability is characterized by the periodic recurrence to
the initial low-density state in a finite optical lattice. We propose a simple
way to retain the localized spatial structures with high atomic concentration,
which may be of interest for applications. Theoretical model, based on the
multiple scale expansion, describes the basic features of the phenomenon.
Results of numerical simulations confirm the analytical predictions.Comment: 17 pages, 13 figure
Class I methanol masers in the outflow of IRAS 16547-4247
The Australia Telescope Compact Array (ATCA) has been used to image class I
methanol masers at 9.9, 25 (a series from J=2 to J=9), 84, 95 and 104 GHz
located in the vicinity of IRAS 16547-4247 (G343.12-0.06), a luminous young
stellar object known to harbour a radio jet. The detected maser emission
consists of a cluster of 6 spots spread over an area of 30 arcsec. Five spots
were detected in only the 84- and 95-GHz transitions (for two spots the 84-GHz
detection is marginal), while the sixth spot shows activity in all 12 observed
transitions. We report the first interferometric observations of the rare 9.9-
and 104-GHz masers. It is shown that the spectra contain a very narrow spike
(<0.03 km/s) and the brightness temperature in these two transitions exceeds
5.3x10^7 and 2.0x10^4 K, respectively. The three most southern maser spots show
a clear association with the shocked gas traced by the H_2 2.12 micron emission
associated with the radio jet and their velocities are close to that of the
molecular core within which the jet is embedded. This fact supports the idea
that the class I masers reside in the interface regions of outflows. Comparison
with OH masers and infrared data reveals a potential discrepancy in the
expected evolutionary state. The presence of the OH masers usually means that
the source is evolved, but the infrared data suggest otherwise. The lack of any
class II methanol maser emission at 6.7 GHz in the source raises an additional
question, is this source too young or too old to have a 6.7 GHz maser? We argue
that both cases are possible and suggest that the evolutionary stage where the
class I masers are active, may last longer and start earlier than when the
class II masers are active. However, it is currently not possible to reveal the
exact evolutionary status of IRAS 16547-4247.Comment: 14 pages, 6 figures, 4 tables, accepted by MNRA
Modulational Instability in Nonlinearity-Managed Optical Media
We investigate analytically, numerically, and experimentally the modulational
instability in a layered, cubically-nonlinear (Kerr) optical medium that
consists of alternating layers of glass and air. We model this setting using a
nonlinear Schr\"odinger (NLS) equation with a piecewise constant nonlinearity
coefficient and conduct a theoretical analysis of its linear stability,
obtaining a Kronig-Penney equation whose forbidden bands correspond to the
modulationally unstable regimes. We find very good {\it quantitative} agreement
between the theoretical analysis of the Kronig-Penney equation, numerical
simulations of the NLS equation, and the experimental results for the
modulational instability. Because of the periodicity in the evolution variable
arising from the layered medium, we find multiple instability regions rather
than just the one that would occur in uniform media.Comment: 13 pages, 12 figures (several with multiple parts); some important
changes from the page proof stage implemented in this preprint versio
Optical Cherenkov radiation by cascaded nonlinear interaction: an efficient source of few-cycle energetic near- to mid-IR pulses
When ultrafast noncritical cascaded second-harmonic generation of energetic
femtosecond pulses occur in a bulk lithium niobate crystal optical Cherenkov
waves are formed in the near- to mid-IR. Numerical simulations show that the
few-cycle solitons radiate Cherenkov (dispersive) waves in the
\lambda=2.2-4.5\mic range when pumping at \lambda_1=1.2-1.8\mic. The exact
phase-matching point depends on the soliton wavelength, and we show that a
simple longpass filter can separate the Cherenkov waves from the solitons. The
Cherenkov waves are born few-cycle with an excellent Gaussian pulse shape, and
the conversion efficiency is up to 25%. Thus, optical Cherenkov waves formed
with cascaded nonlinearities could become an efficient source of energetic
near- to mid-IR few-cycle pulses.Comment: Extended version of Nonlinear Optics 2011 contribution
http://www.opticsinfobase.org/abstract.cfm?URI=NLO-2011-NTuA7. Submitted for
Optics Express special issue for NLO conferenc
How do methanol masers manage to appear in the youngest star vicinities and isolated molecular clumps?
General characteristics of methanol (CH3OH) maser emission are summarized. It
is shown that methanol maser sources are concentrated in the spiral arms. Most
of the methanol maser sources from the Perseus arm are associated with embedded
stellar clusters and a considerable portion is situated close to compact HII
regions. Almost 1/3 of the Perseus Arm sources lie at the edges of optically
identified HII regions which means that massive star formation in the Perseus
Arm is to a great extent triggered by local phenomena. A multiline analysis of
the methanol masers allows us to determine the physical parameters in the
regions of maser formation. Maser modelling shows that class II methanol masers
can be pumped by the radiation of the warm dust as well as by free-free
emission of a hypercompact region hcHII with a turnover frequency exceeding 100
GHz. Methanol masers of both classes can reside in the vicinity of hcHIIs.
Modelling shows that periodic changes of maser fluxes can be reproduced by
variations of the dust temperature by a few percent which may be caused by
variations in the brightness of the central young stellar object reflecting the
character of the accretion process. Sensitive observations have shown that the
masers with low flux densities can still have considerable amplification
factors. The analysis of class I maser surveys allows us to identify four
distinct regimes that differ by the series of their brightest lines.Comment: 8 pages, 4 figures, invited presentation at IAU242 "Astrophysical
Masers and their environments
Modulational instability in periodic quadratic nonlinear materials
We investigate the modulational instability of plane waves in quadratic
nonlinear materials with linear and nonlinear quasi-phase-matching gratings.
Exact Floquet calculations, confirmed by numerical simulations, show that the
periodicity can drastically alter the gain spectrum but never completely
removes the instability. The low-frequency part of the gain spectrum is
accurately predicted by an averaged theory and disappears for certain gratings.
The high-frequency part is related to the inherent gain of the homogeneous
non-phase-matched material and is a consistent spectral feature.Comment: 4 pages, 7 figures corrected minor misprint
Modulational instability of bright solitary waves in incoherently coupled nonlinear Schr\"odinger equations
We present a detailed analysis of the modulational instability (MI) of
ground-state bright solitary solutions of two incoherently coupled nonlinear
Schr\"odinger equations. Varying the relative strength of cross-phase and
self-phase effects we show existence and origin of four branches of MI of the
two-wave solitary solutions. We give a physical interpretation of our results
in terms of the group velocity dispersion (GVD) induced polarization dynamics
of spatial solitary waves. In particular, we show that in media with normal GVD
spatial symmetry breaking changes to polarization symmetry breaking when the
relative strength of the cross-phase modulation exceeds a certain threshold
value. The analytical and numerical stability analyses are fully supported by
an extensive series of numerical simulations of the full model.Comment: Physical Review E, July, 199
Methanol in W3(H2O) and Surrounding Regions
We present the results of an interferometric study of 38 millimeter-wave
lines of CH3OH in the region around the water maser source W3(H2O) and a region
extending about 30" to the south and west of the hydroxyl maser source W3(OH).
The methanol emitting region around W3(H2O) has an extent of 2.0" x 1.2"
(4400 x 2600 AU). The density is of order 1.e7 cm-3, sufficient to thermalize
most of the methanol lines. The kinetic temperature is approximately 140 K and
the methanol fractional abundance greater than 1.e-6, indicative of a high
degree of grain mantle evaporation. The W3(H2O) source contains sub-structure,
with peaks corresponding to the TW source and Wyrowski's B/C, separated by 2500
AU in projection. The kinematics are consistent with these being distinct
protostellar cores in a wide binary orbit and a dynamical mass for the region
of a few tens of Mo.
The extended methanol emission to the southwest of W3(OH) is seen strongly
only from the lowest excitation lines and from lines known elsewhere to be
class I methanol masers, namely the 84.5 GHz 5(-1)-4(0)E and 95.2 GHz
8(0)-7(1)A+ lines. Within this region there are two compact clumps, which we
denote as swA and swB, each about 15" (0.16 pc projected distance) away from
W3(OH). Excitation analysis of these clumps indicates the presence of lines
with inverted populations but only weak amplification. The sources swA and swB
appear to have kinetic temperatures of order 50-100 K and densities of order
1.e5 - 1.e6 cm-3. The methanol fractional abundance for the warmer clump is of
order 1.e-7, suggestive of partial grain mantle evaporation. The clumping
occurs on mass scales of order 1 Mo.Comment: 28 pages including 6 figures and 4 tables, accepted by Ap
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